The present invention relates to a method for increasing the rated capacity of a commercial oxidation reactor. More particularly, the present invention is a method for debottlenecking a commercial reactor system of the type used for catalytic liquid phase oxidation of paraxylene to produce terephthalic acid.
Practically all terephthalic acid is produced on a commercial scale by catalytic, liquid phase, air oxidation of paraxylene. Commercial processes use acetic acid as a solvent and a multivalent heavy metal or metals as catalyst. Cobalt and manganese are the most widely used heavy metal catalysts, and bromine is used as a renewable source of free radicals in the process.
In conventional air-oxidation processes, acetic acid solvent, air (molecular oxygen), paraxylene and catalyst are fed continuously into a back-mixed oxidation reactor that is maintained at from 150° C. to 225° C. and from about 500 to 2,500 kPa (i.e., 5–25 atm). In such conventional oxidation reactors, the feed solvent:paraxylene mass ratio is typically less than 5:1. Air is added to the reactor in amounts in excess of the stoichiometric requirements for full conversion of the paraxylene to terephthalic acid to minimize formation of undesirable by-products, such as color formers. The oxidation reaction is exothermic, and heat is removed from the reactor by allowing the acetic acid solvent to vaporize. The corresponding vapor is condensed and most of the condensate is refluxed to the reactor, with some condensate being withdrawn to control water concentration in the system (two moles of water are formed per mole of paraxylene reacted to terephthalic acid). The reactor residence time is typically 30 minutes to 2 hours, depending on the process. Depending on oxidation reactor operating conditions, e.g., temperature, catalyst concentration and residence time, significant degradation of the solvent and precursor can occur, which, in turn, can increase the cost of operating the process. Such conventional oxidation reactors are well known, for example, see U.S. Pat. No. 5,099,064.
The effluent, i.e., reaction product, from the oxidation reactor is a slurry of crude terephthalic acid (TA) crystals in acetic acid. A significant and undesirable impurity in the crude TA is 4-carboxybenzaldehyde (4-CBA), which is incompletely oxidized paraxylene, although p-tolualdehyde and p-toluic acid can also be present along with undesirable color formers. The slurry of crude terephthalic acid crystals is further processed (e.g., purified by post-oxidation and/or hydrogenation) and recovered (e.g., by filtration, washing and drying) according to established methods.
The present invention provides a reliable and affordable method to increase the production capacity of a conventional terephthalic acid process by up to 100% by increasing the capacity of the oxidation reactor system, i.e., “debottlenecking” the reactor system. Debottlenecking is achieved according to the invention by effectively staging the oxidation reaction utilizing a first reaction zone, i.e., first reactor, followed by a second reaction zone, i.e., the existing, conventional reactor.